High frequency coaxial filter

ABSTRACT

A high frequency coaxial line filter is disclosed. The filter includes a section of coaxial line containing an input resonator and an output resonator decoupled by a decoupling capacitive structure disposed therebetween. The resonators each include an inductor having a central hub with radially directed inductive spokes extending from the hub to the outer conductor of the coaxial line. Capacitors for the circuit are formed by capacitive gaps formed between the ends of the hub portions of the inductors and adjacent conductive structure.

te States Patent 2,543,721 2/1951 Collard et a]. 333/73 UX 2,603,707 7/1952 .laynes 333/73 C 2,798,206 7/1957 Baroch 333/73 C 3,335,314 8/1967 Espinosa ct al. 315/35 X 3,353,121 11/1967 Dube 3l5/3.5 X 3,366,897 1/1968 Konrad 3l5/3.5 X

Primary Examiner-Herman Karl Saalbach Assistant Examiner-Marvin Nussbaum Alt0rneys Stanley Z. Cole and Gerald L. Moore ABSTRACT: A high frequency coaxial line filter is disclosed. The filter includes avsection of coaxial line containing an input resonator and an output resonator decoupled by a decoupling capacitive structure disposed therebetween. The resonators each include an inductor having a central hub with radially directed inductive spokes extending from the hub to the outer conductor of the coaxial line. Capacitors for the circuit are formed by capacitive gaps formed between the ends of the hub portions of the inductors and adjacent conductive structure.

PATENTEUAUBBI l9?! 3.602.848

FIGJ

a 1 9 9 THOMASI EF L E OmARD .F' '/./////A BEWMM 0'5 6 ATTORNEY HIGH FREQUENCY COAXIAL FILTER DESCRIPTION OF TI-IEPRIOR ART I Heretofore, coaxial band-pass filters for high-frequency circuits, i.e., S-band to Cband,have been designed from an impedance basis using the specific values of inductance and capacitance derived from a Butterworth or Chebishev filter solution for maximally flat amplitude response. Filters of this type at these frequencies employ inductors fonned by sections of coaxial line sufficiently long to provide substantial distributed inductance, whereas the capacitors are quite small. Such filters are difficult to fabricate at these high frequencies.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved high-frequency coaxial filter.

One feature of the present invention is the provision of inductors for the resonators of the filter circuit which each include a central conductive hub portion having a plurality of radially directed conductive spokes extending from the hub to the outer conductor of the coaxial line, the number of such spokes and their dimensions being chosen toprovide the value of inductance required for the inductors of the coaxial filter circuit, whereby the relatively small inductance of the inductor may be accurately controlled with relative ease.

Another feature of the present invention is the same as the preceding feature wherein the series capacitors of the circuit are;formed by capacitive gaps between the ends of the hub portions of the inductors and adjacent conductive structure, whereby the relatively large values of capacitance required for the filter circuit are accurately controlled with relative ease.

Another feature of the present invention is the same as any one or more of the preceding features wherein a relatively large shunt capacitor for decoupling the resonators of the circuit is formed by a conductive discstructure insulatively transversely disposed within the outer conductor of the coaxial line with the shunt capacitance formed by a capacitive gap between the outer periphery of the disc and the adjacent inner wall of the outer conductor.

BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the present .invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic equivalent circuit diagram of a bandpass coaxial filter of the present invention,

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I, there is shown a schematic lumped element equivalent circuit 1 for the band-pass coaxial line filter of the present invention. The filter I includes an input resonator formed by inductor L and capacitor C, tuned for a resonant frequency to define one pole of the filter circuit. The inductor L, and capacitor C, are connected together in a closed resonant circuit loop 2 via the intermediary of a relatively large bypass capacitor C and ground. An output resonator is formed by inductor L and capacitor C2 tuned for a resonant frequency to define a second pole of the filter I. The second inductor L and second capacitor C are connected together to form a closed resonant circuit loop 3 via the intermediary of the large bypass capacitor C, and ground.

An input capacitor C is connected to the input resonator loop 2 for matching the impedance of an input device, such as a source of microwave energy or a transmission line, not shown, to the input resonator 2. The input device or transmission line is connected at terminals 4. An output capacitor C is connected to the output resonator 3 for matching the impedance of the output resonator 3 to a suitable utilization device or transmission line, not shown, connected at output terminals 5.

The specific values of the elements L,, C,, L C, and C, are found by reference to any one of a number of texts on bandpass filter design taking into consideration the desired characteristics of the filter, such as operating frequency, width of the passband, amplitude response, phase shift, etc. The solutions to be employed are those derived on the basis of circuit admittance, as opposed to those solutions based on circuit impedances. Typical solutions for a maximally fiat amplitude and phase response based on admittance are to be found in a text titled, Microwave Filters, Impedance Matching Networks and Coupling Structures by Matthaei, Young & Jones, published by McGraw-Hill Book Company in 1964, at pages 83-162. Such solutions are characterized by the inductors L, and L of the resonators 2 and 3, having relatively small values of inductance, whereas the capacitors C, and C of the resonators 2 and 3 have relatively large values of capacitance for circuits at relatively high operating frequencies, i.e., above L-band. The capacitance of C, will be large compared to the capacitance of either C, or C C and C will normally have values of capacitance small compared to the capacitance of either C, or C Referring now to FIGS. 2 and 3, there is shown a physical realization of the circuit of FIG. 1 for operation at relatively high frequencies, such as L-band to above C-band. The coaxial filter includes an outer tubular conductor 6 defining the ground plane of the filter. The inductors L, and L of the input and output resonators 2 and 3, respectively, include a centrally disposed hub portion 7, as of copper, having a plurality of radially directed spoke members 8, as of copper, extending from the hub 7 to the outer conductor 6. The spokes 8 define the inductance of the inductor. The inductance of each spoke 8 is determined by its cross-sectional dimensions and length. The inductances L of the spokes 8 are in parallel and, thus, the total inductance L of the inductor is found by the expression: 1/L,=1/L ,+l/L +l/L l/L where L L L ...L are the inductances of the respective individual spokes 8. By forming the inductors as spokes 8 on the hub member 7, accurate control over the inductance of inductors L, and L is readily achieved. This becomes especially important above C- band.

The bypass or decoupling capacitor C is formed by a conductive disc 9, as of copper, which is transversely insulatively disposed within the outer conductor 6. The relatively large capacitance of the capacitor C is formed by an annular capacitive gap, at the outer periphery of the disc 9 between the mutually opposed surfaces of the disc 9 and the inner surfaces of an annular recess 11 in the inside wall of the outer conductor 6 into which the disc 9 extends radially. An insula tive retaining ring structure 12, as ofTeflon, holds the disc 9 in position within the recess II.

The capacitors C,and C of the resonators 2 and 3, respectively, are formed by capacitive gaps C, and C,, respectively, between the mutually opposed surfaces of the ends of the hub portions 7 and the adjacent surface of the conductive disc 9. The value of capacitance for these capacitors is readily changed by changing the gap spacing or by changing the mutually opposed surface areas by changing the cross-sectional dimensions of the hub portions 7, at their ends.

Capacitors C, and C are formed by capacitive gaps C, and C respectively, between the outer ends of hub portions 7 of the inductors and the adjacent opposed end areas of the center conductors l3 and 14 of coaxial transmission lines connected at terminals 4 and 5, respectively.

' Referring now to FIGS. 4 and 5, there is shown an altcrnative embodiment of the present invention. The equivalent lumped circuit diagram, as shown in FIG. 4, is substantially identical to the diagram of FIG. I with the exception that a third resonator, indicated by circuit loops 21, and a second a bypass capacitor C have been added between resonators 2 and 3. The third resonator 21 is formed by inductor L resonated with the capacitance of capacitors C and C The resonator 21 is tuned for a resonant frequency to define a third pole of the filter circuit.

Referring now to FlG. 5, there is shown the physical realization of the circuit of FIG. 4. The physical realization is identical to that of FIGS. 2 and 3 with the exception that it has been modified to incorporate the additional capacitor C and resonator 21. More particularly, the third inductor L is formed by a central hub 7 having spokes 8 interconnected to the outer conductor 6. Theinductor L is positioned betweenbypass capacitor C and the output inductor L THe sccond bypass capacitor C is formed by a transversely insulatively disposed conductive disc 9 substantially identical to capacitor C and is disposed between inductor L: and the output inductor L Capacitors C and C are formed by capacitive gaps between the ends of the hub portion 7 of inductor L and the adjacent capacitive disc 9 of capacitors C and C respectively. If desired, additional filter poles may be provided by adding additional resonators and bypass capacitors into the filter circuit in between the input and output resonators 2 and 3.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. in a high-frequency coaxial filter circuit, means forming a length of tubular conductor forming the outer conductor of the coaxial line filter, means forming an input resonator disposed within said outer conductor and having a first capacitor and a first inductor tuned for a resonant frequency to define one pole of the filter, means forming an output resonator disposed within said outer conductor and having a second capacitor and a second inductor tuned for a resonant frequency to define a second pole of the filter, means forming a decoupling capacitor structure disposed within said outer conductor intermediate said input and output resonators for decoupling said resonators, means for coupling wave energy into said input resonator and out of said output resonator, THE IMPROVEMENT WHEREIN, said first and second in ductors each comprise a cylindrical conductive hub portion generally centrally disposed of said surrounding outer conductor with a plurality of conductive spoke members extending radially from said hub to said outer conductor, said cylindrical hub portion having a greater axial length than the thickness of said conductive spoke members. said decoupling capacitor structure comprising a conductive disc insulatively transversely disposed within said outer conductor, with the mutually opposed surface between said capacitor structure and the inside surface of said outer conductor near the outer periphery of said conductive disc defining the decoupling capacitance of said decoupling capacitor structure, said hub portions of said inductors being insulatively spaced at their ends from said conductive disc to define said first and second capacitors by the mutually opposed area between the ends of said hub portions and said conductive decoupling capacitor structure.

2. The apparatus of claim 1 wherein said input and output coupling means includes fourth and fifth capacitors, a pair of inner conductors disposed generally centrally within said outer conductor at opposite ends of said input and output resonators adjacent said hub portions of said first and second inductors for coupling wave energy into and out of the filter, said fourth and fifth capacitors being defined by the mutually opposed surface area between the ends of said pair of inner conductors and said adjacent hub portions of said first and second inductors.

3. The apparatus of claim 1 wherein said outer conductor has a radially outwardly directed recess at its inner wall, and

wherein said conductive disc extends radially into said recess.

4. The apparatus ofclalm 1 including means forming a third resonator disposed within said outer conductor intermediate said input and output resonators, said third resonator including a third inductor and a pair of resonator capacitors defining the resonant frequency thereof, said third resonator being tuned for a resonant frequency to define a third pole of the filter, said third inductor including a conductive hub portion generally centrally disposed of said outer conductor with a plurality of conductive spoke members extending radially from said hub to said outer conductor, said hub portion of said third inductor being insulatively spaced at opposite ends from said decoupling capacitor structure to define said pair of third resonator capacitors by the mutually opposed surface area between the ends of said hub portion and said conductive decoupling capacitor structure. 

1. In a high-frequency coaxial filter circuit, means forming a length of tubular conductor forming the outer conductor of the coaxial line filter, means forming an input resonator disposed within said outer conductor and having a first capacitor and a first inductor tuned for a resonant frequency to define one pole of the filter, means forming an output resonator disposed within said outer conductor and having a second capacitor and a second inductor tuned for a resonant frequency to define a second pole of the filter, means forming a decoupling capacitor structure disposed within said outer conductor intermediate said input and output resonators for decoupling said resonators, means for coupling wave energy into said input resonator and out of said output resonator, THE IMPROVEMENT WHEREIN, said first and second inductors each comprise a cylindrical conductive hub portion generally centrally disposed of said surrounding outer conductor with a plurality of conductive spoke members extending radially from said hub to said outer conductor, said cylindrical hub portion having a greater axial length than the thickness of said conductive spoke members, said decoupling capacitor structure comprising a conductive disc insulatively transversely disposed within said outer conductor, with the mutually opposed surface between said capacitor structure and the inside surface of said outer conductor near the outer periphery of said conductive disc defining the decoupling capacitance of said decoupling capacitor structure, said hub portions of said inductors being insulatively spaced at their ends from said conductive disc to define said first and second capacitors by the mutually opposed area between the ends of said hub portions and said conductive decoupling capacitor structure.
 2. The apparatus of claim 1 wherein said input and output coupling means includes fourth and fifth capacitors, a pair of inner conductors disposed generally centrally within said outer conductor at opposite ends of said input and output resonators adjacent said hub portions of said first and second inductors for coupling wave energy into and out of the filter, said fourth and fifth capacitors being defined by the mutually opposed surface area between the ends of said pair of inner conductors and said adjacent hub portions of said first and second inductors.
 3. The apparatus of claim 1 wherein said outer conductor has a radially outwardly directed recess at its inner wall, and wherein said conductive disc extends radially into said recess.
 4. The apparatus of claim 1 including means forming a third resonator disposed within said outer conductor intermediate said input and output resonators, said third resonator including a third inductor and a pair of resonator capacitors defining the resonant frequency thereof, said third resonator being tuned for a resonant frequency to define a third pole of the filter, said third inductor including a conductive hub portion generally centrally disposed of said outer conductor with a plurality of conductive spoke members extending radially from said hub to said outer conductor, said hub portion of said third inductor being insulatively spaced at opposite ends from said decoupling capacitor structure to define said pair of third resonator capacitors by the mutually opposed surface area between the ends of said hub portion and said conductive decoupling capacitor structure. 